ECE 35 Lab 1_V17.pdf

ECE 35 Lab 1 Introduction to Analog Circuits

This lab will illustrate some of the differences between analog and digital circuits. There are assignments for this first lab, A & B. In part A we design a circuit to produce an arbitrary voltage, rather than the digital true or false voltages. In part B we will observe that voltages do not change instantaneously, as one imagines in a digital circuit, and measure the rise and fall times.

Before starting this lab:

You are required to do the following before starting the lab. All documents are found on the course website.

1. Read the “Lab Syllabus” and “Lab Write-up Tips”

2. Read the “Equipment Introduction”. We will use all the instruments in this introduction in this lab.

3. Do the prelab (analysis for Part A) in advance and be able to show the TA your result 4. Have these ready for all labs:

 USB thumb/flash drive, to store and transfer your data.  ACMS printing account to print in the labs

(http://acms.ucsd.edu/students/print/index.html).

Prelab:

You are required to read the corresponding lab assignment before coming to lab. Every student must individually complete the tasks and questions listed in the Prelab section on a piece of letter-sized paper and turn it in to your TA at the beginning of the lab. You will forfeit your prelab credits if you turn in late or to the wrong lab section. Prelab for this project is to complete the analysis in part A.-1.

A. Design of a Voltage Reference Circuit:

The objective of this project is to design and test a voltage reference circuit which creates an open circuit voltage of 2.5 V ± 50 mV, from two 5 V supplies. The circuit is to be made with resistors and the constraint is that it should draw between 1 and 5 mA current from the supplies.

1. Analysis: The circuit diagram is shown to the right. Both batteries are 5V.

 Find the values of R1 and R2 that will give an output voltage of 𝑉𝑜𝑢𝑡 = 2.5𝑉, and a supply current of 1 mA.

 Find the values that will give the correct output voltage with a supply current of 5 mA.

2. Components: It is hard to find resistors that have exactly your desired values in the lab, because they are manufactured only in certain standard values (and with certain standard accuracies). However it is easy to obtain multiple resistors with nearly the same values.

In prelab you calculated the upper and lower limits of your resistors that satisfy the specifications. In this case we can use 4 resistors of the same (nominal) value, somewhere between the limits you found in part 1. One will form R1 and the other three in series will form R2. Pick four resistors from lab that satisfy these constraints. Measure them all with a

multimeter. The actual resistances should be within 1% of the nominal value. Average the upper and lower limits for R1. Use the measured resistance closest to this average for R1, and use the series connection of the other three resistors to form R2.

Now calculate 𝑉_𝑜𝑢𝑡 using these actual (not nominal) resistances. If the calculated value is 2.5 V ± 50 mV, go to the next step. If not, your resistors are not within 1% or you have made an error in your calculation. Get it right before you continue.

3. Construction: Build the circuit on your breadboard. Use two power supplies and connect to the ports marked ± 20 V. Adjust both output voltages to be ±5 V. Then measure the current through your circuit. Remember, current

measurements need to be done in series with the circuit. Connecting the leads in parallel with a resistor will cause a short; explain in your report why it would cause a short. Is it what you expected when you take your measurement? If the current is not zero but not correct, you have made a mistake somewhere.

When you have the supply current right, measure the output voltage. Remember, the voltmeter measures the voltage difference of two points, and it is connected in parallel to the circuit. Connect the leads correctly and it will give you the 𝑉𝑜𝑢𝑡. Is it within spec? If so, show the TA your measurements and have your signature sheet signed off to move to the next step.

4. Effect of loading on voltage divider performance: Congratulations on your first circuit! However, notice that there is nothing connected to your circuit output (open circuit). Even though you successfully measured a 𝑉_𝑜𝑢𝑡 = 2.5𝑉 for an

open circuit, in practice the output of a circuit can be affected by what is connected to this output (called

loading effect). In this part of the lab you’ll use a modified voltage divider circuit to investigate the effect of any resistor connected between the output and ground. The circuit is a simplified version of the one on page 1, with only a single 4V voltage source. R1 and R2 remain the same values. Without any load resistor in place (so-called

Add an R-Load resistor in parallel with and has a value much larger than R2, let’s make it R-Load = 40 kΩ. Now connect R-Load between 𝑉_𝑜𝑢𝑡 and ground. Using a multimeter, measure 𝑉_𝑜𝑢𝑡 and the current 𝐼_𝑜𝑢𝑡 through the resistor R-Load. Does 𝑉_𝑜𝑢𝑡 deviate from its open-circuit value? Discuss the reason in your lab report.

Now reduce R-Load to 30 kΩ, measure and record 𝑉_𝑜𝑢𝑡 and 𝐼_𝑜𝑢𝑡 again. Notice how much the deviation between 𝑉_𝑜𝑢𝑡 and its open circuit value change. Keep decreasing R-Load to 20, 10 and then 5 kΩ. For each choice of R-Load, plot your collected sets of data points with the x-axis being 𝐼_𝑜𝑢𝑡 and y-axis 𝑉_𝑜𝑢𝑡. Fit a linear curve through these points. In your lab report discuss answers to these questions:

1. What is the y-axis intercept (𝑉𝑜𝑢𝑡) of the curve at 𝐼𝑜𝑢𝑡 = 0𝐴?

2. Discuss the difference of 𝑉𝑜𝑢𝑡 between no load (open circuit) and when the output is loaded with a resistor (𝐼𝑜𝑢𝑡 > 0𝐴). This is called the loading effect.

3. What is the value of the slope? How does the slope relate to the R1 and R2 values in your circuit according to your calculations? (Hint: the equation of your line from the graph is Vout as a function of Iout. Start with voltage division of the circuit and Ohm’s law for RL. Eliminate RL to derive an equation in the same form as the line).

4. Discuss the limitation on the range of R-Load one can connect to this circuit if we require 𝑉𝑜𝑢𝑡 to deviate from its open circuit value by no more than 5%.

We will revisit the topic of circuit loading later this quarter when we introduce operational amplifiers, which are a very important type of circuit that actively compensate for these loading effect, thus effectively ‘isolating’ the output and making sure it’s always the predicted value regardless of how much the load is.

B. Design of a Voltage divider for Digital Signals:

Use the function generator as our power source, and set it to give a 0 to +5 V square wave at a frequency of 10 kHz. To check the function generator settings we will use the O-scope and verify we have the desired signal. Get the O-scope probe and the function generator lead and connect them to their respective machines. Connect the large center lead of the O-scope to the red lead of the function generator, and the small black leads together, as indicated below.

The O-scope display will show you the signal you set on the function generator. If you set them correctly you should get a display similar to below.

The small arrow with a “1” to the left indicates where zero volts is at. If your wave goes below this arrow then your DC offset on the function generator setting is wrong and should be adjusted before moving on. Also check the channel 1 menu to confirm that the coupling is DC, the BW limit is OFF; and Probe is 10X. Once the settings are verified, connect your function generator leads to each end of your circuit. Connect the O-scope lead to 𝑉_𝑜𝑢𝑡, and the black lead to

Sometimes digital signals do not switch instantaneously between high and low like an ideal square wave does. Instead signals take some time to rise up or fall down. It is important to observe and understand the rise and fall times

τ

R and

τ

F. These are usually measured as the time

for the output to change between 10% and 90% of the final voltage. We will measure

t

R and

t

F at the output of the voltage divider with the oscilloscope using the graph on top as a guide. Refer back to the oscilloscope section of “Equipment Introduction” to guide you through the next steps.

To get a graph like above, you will have to zoom in. Adjust the two knobs labeled “scale” on the O-scope, one scales vertically and the other scales horizontally. You may have to re-adjust the position of the wave with the knobs labeled “position.” Zoom in enough so you can see either the rise or fall of the wave separately and clearly on the scope display.

To measure

t

R or

t

F of the output square wave we will use the “cursor” option of the O-scope. Press the button labeled “cursor” and change the type to “Time.” Make sure the source is “CH1.” Two Cursors will appear, they will give you a time and voltage reading. Move one to 10%

V

out

and 90%

V

out. Below the source menu you will see “Δ

t

,” it is calculating the time difference

where you placed the cursors. The figure below is an example of how to measure with the cursor.

Use your USB thumb drive to save the scope trace showing these values, for both rise and fall. Include all scope traces in your lab report. Show the TA your traces and have them sign your signature sheet.

Discuss in your lab report:

 How is a voltage meter and a current meter modeled when making measurements  The definition of a short circuit and how it effects a circuit if it was introduced into a

circuit by mistake

 The definition of an open circuit and why is this important to have open circuit conditions when making voltage measurements